1. Field of the Invention
The present invention relates to a camera system operable by means of a cooperation between a camera body and a camera accessory, such as an interchangeable lens or lens converter, to be mounted on the camera body. It also relates to an improvement of a camera body itself and a camera accessory itself to be employed in the camera system.
2. Description of the Prior Art
According to the prior art camera system operable by means of a cooperation between a camera body and a camera accessory, the camera accessory, such as an interchangeable lens, has an information carrying circuit, such as a ROM (read-only-memory), wherein various fixed data, for example, maximum and minimum aperture size of the interchangeable lens, is previously stored at various addresses. When a particular address is specified, data stored therein is read out and sent to the camera body.
However, the prior art camera systems have various problems when designing the camera system for the practical use. One problem is the mechanical connection between the camera body and camera accessory. Since the camera accessories are vary often mounted on and dismounted from the camera body, there exists a chance of an incompleted connection which fails in realizing an expected cooperation therebetween. For example, the setting of aperture size by means of rotating the aperture setting ring of the interchangeable lens would not be correctly transmitted to the camera body if the rotational mounting of the interchangeable lens on the camera body is incomplete and the rotational relation therebetween is incorrect.
Another problem, which relates to the above problem, is the electric connection between the camera body and camera accessory. To exchange the information between the camera body and camera accessory, a plurality of terminals are necessary both on the camera body and on the camera accessory, but the number of the terminals should be as small as possible to avoid the possibility of any misconnection. According to the prior art, however, a consideration of such a problem relating to a practical product has been insufficient.
A further problem is how to deal with a signal representing a variant data, such as an aperture size data, to be set in the camera accessory and to be transmitted to the camera body. Some contrivances are necessary both in the camera accessory side and the camera body side to have the camera body side accurately respond to every change in the variant data on the camera accessory side.
On the other hand, a prior art camera system has been known which includes: an electric resistive plate provided in a camera body; and a movable contact member provided in the camera body, which movable contact member is slidable along the resistive plate and moves relative to the rotation of an aperture control ring provided on the lens; whereby by changing the resistance value as the shift of the contact member on the resistive plate, a signal relative to the set aperture size is obtained. According to the above prior art arrangement, the resistivity of the resistive plate varies between the manufactured pieces, and also after having been used overtime. Thus, its reliability is very poor. Furthermore, in the case where the calculation is carried out in digital form, it is necessary to change the obtained signal into digital form and, for this purpose, the prior art arrangement further requires an A-D (analog-to-digital) converter, which not only increases the constucting components, but also requires an A-D conversion time which delays the shutter release operation; one may miss a shutter chance.
To solve to above problem of the arrangement for producing a signal representing a variant data, one may think of replacing the resistive plate with a coded pattern which directly produces a digital signal as the contact member slides along the coded pattern. Since this arrangement, however, obtains the digital signal that changes discretely directly from the coded pattern, the coded pattern becomes rather large in area and complicated in pattern layout to obtain an electric signal with a high preciseness. Therefore, the increase of area of the coded pattern results in bulky size of a camera; and the complicated pattern layout results in difficult manufacturing of the coded pattern and may easily produce an erroneous signal by a small shock to the device or by a slight displacement of the contact member.
In addition to the above, the prior art arrangement for producing a signal representing a variant data, such as an aperture data, has a following disadvantage particularly when obtaining a signal representing an amount of stop-down, i.e., a difference between the maximum aperture size and set aperture size, from the arrangement.
Generally, the signal representing an amount of stop-down can be obtained by the steps of obtaining maximum aperture size and set aperture size, and subtracting the set aperture size from the maximum aperture size. Or, it can be obtained by the steps of obtaining an amount of shift effected to the aperture setting element for the aperture change from the maximum to set aperture size, and converting the shifted amount to stop-down amount.
According to the former method, it is necessary to provide a set of coded pattern and movable contact member both for obtaining the maximum aperture size and for obtaining the set aperture size. In addition, the former method further requires a subtractor. This results in an increase of constructing parts and bulky size.
According to the latter method, since the aperture size is generally given by a multiple of 1/2 Av (Av represents an aperture size under APEX numbering system and, here "1/2 Av" means "1/2 in Av value"), the set aperture size is calculated using the signal representing the maximum aperture size and the signal representing the shifted amount which is equal to the multiple of 1/2 Av. When the mounted lens is a standard type having a maximum aperture size equal to a standard F-stop value, such as F1.4, F1.7, F2, F2.8, F3.4, F4, or the like having a value equal to the multiple of 1/2 Av, the set aperture size as calculated also has a value equal to the multiple of 1/2 Av, i.e., equal to the standard F-stop value. A problem arises when the mounted lens is a non-standard type having a maximum aperture size other than standard F-stop values, such as F1.8, F2.5, F3.5 or the like. When such a non-standard type lens is mounted, the calculated set aperture size is not equal to the multiple of 1/2 Av, but includes an error less than 1/2 Av. To reduce such an error, it is necessary to give the aperture value by a multiple of a smaller fraction of 1 Av, e.g., the error would be less than 1/4 Av if the set aperture value is given by a multiple of 1/4 Av. This can be practiced by means of subdividing the patterns on the digital code plate, which however increases the size of the digital code plate and the number of patterns on it. This problem also arises in the former method.